The present invention relates to a monolithic microwave mixer circuit of the type in which diodes and a circuit are integrated onto a semiconductor insulating substrate.
A balanced type mixer using a couple of mixer diodes has been widely used in this field. This type of mixer can better separate a signal from a local oscillation signal, and suffers from little noise figure degradation due to the noise of the local oscillator.
The balanced type mixer requires a phase difference of 180.degree. between the signal and the local oscillation signal at the two mixer diodes. To have such phase difference, the mixer is provided with a phase inverting circuit.
FIG. 1 is a schematic illustration of conventional balanced type microwave mixers using a rat-race ring-type power-distributing circuit. A rat-race ring-type power-distributing circuit 1 is provided with a ring-like electric path 1a with a circular length equal to 6/4 of a wavelength .lambda..sub.0 of an input signal. The power distributing circuit is further provided with a terminal 4 of a mixer diode 3 distanced 3/4.lambda..sub.0 from a signal input terminal 2 as viewed counterclockwise. Additionally used is a terminal 6 of the mixer diode 5 distanced 1/4.lambda..sub.0 from the signal input terminal 32 as viewed clockwise. An input terminal 7 for a local oscillation signal is centered between the terminals 4 and 6 of the mixer diodes 3 and 5 with a distance of 1/4.lambda..sub.0 from each of those terminals 4 and 6.
The local oscillation signal input through the terminal 7 is applied to and reaches the two mixer diodes 3 and 5 in-phase. The signal input through the terminal 2 reaches the mixer diodes 3 and 5 at phases different from each other by 180.degree.. The phase relationship of the applied signals enables the circuit to operate in a balanced mode. The mixer diode 5 is forwardly connected, while the diode 3 is reversely connected. The outputs of these diodes are connected together and to a low-pass filter 8. The output of the low-pass filter 8 is derived from an output terminal 9 for intermediate frequency.
FIG. 2 is a schematic illustration of the balanced type microwave mixer using branch-line coupler. This mixer employs a frame shaped line having a pair of parallel arranged lines 11 and 12 of 0.25.lambda..sub.0 long and a characteristic impedance Z.sub.0, and another pair of lines 13 and 14 of 0.25.lambda..sub.0 long and a characteristic impedance Z.sub.0 /.sqroot.2, which are arranged perpendicular to and connected to both ends to the pair of lines 11 and 12, as shown. The ends of the lines 13 and 14 are respectively used as a signal input terminal 15 and an oscillation signal input terminal 16. The other ends of those lines 13 and 14 are respectively used as a terminal 18 for a mixer diode 17 and a terminal 20 for a mixer diode 19. The mixer diode 17 is reversely connected between the terminal 18 and the low-pass filter 21. The mixer diode 19 is forwardly connected between the terminal 20 and the low-pass filter 21. The output of the low-pass filter 21 is derived from an intermediate frequency output terminal 22. In this branch-line coupler, four terminals may be branched from a loop, not four lines.
In the power distributing circuit based on the branch-line coupler, a signal input through the terminal 15 reaches the diode 19 with a delay of 90.degree. after it reaches the diode 17. The signal input through the terminal 16 reaches the diode 17 delayed 90.degree. after it reaches the diode 19. Accordingly, the intermediate frequency signals generated at the diodes 17 and 19 are at phases different from each other by 180.degree., and thus operate as a balanced type microwave mixer.
Since both the circuits of FIGS. 1 and 2 use distributed circuits in the power distributing circuit, the entire length of the lines is long; 3/2.lambda..sub.0 for the rat-race ring-type circuit and .lambda..sub.0 for the branch-line coupler circuit. This makes the circuit large in size as a whole. Particularly in a monolithic microwave integrated circuit in which diodes, a power distributing circuit, and an impedance matching circuit are integrated onto a semiconductor substrate made of GaAs (gallium arsenide), the cost reduction requirement demands the smallest possible pattern and chip. In spite of such a demand, the conventional balanced type mixer inherently has a large chip and is costly.